1. Field of the Invention
The present invention relates to a wideband spontaneous emission light source that employs a light source of spontaneous emission light generated from a rare earth-doped optical fiber and is suitable for use as a light source in a wavelength multiplex optical communication system, optical measurement and so on.
2. Prior Art
A rare earth-doped optical fiber has an amplification characteristic capable of yielding a high gain at a certain wavelength with incident pumping light within a certain range of wavelength. Therefore, if a signal light in a wavelength band in which the rare earth-doped optical fiber has a certain amplification gain is transmitted through the fiber, then the light intensity of the signal light is remarkably increased. This characteristic is currently widely utilized as an optical amplifier in the field of optical communications.
Upon receiving an incident pumping light, the rare earth-doped optical fiber yields a gain of the signal light and also generates a spontaneous emission light. The generated spontaneous emission light is amplified by the gain, and the optical output is increased. The light generated as described above is referred to as Amplified Spontaneous Emission light, which will hereinafter be abbreviated to ASE light.
The rare earth-doped optical fiber is able to produce a large output of ASE light emission by virtue of the high gain possessed by itself and to be used as a wideband light source. In recent years, according to the demand for increasing the communication capacity, there has been examined a wavelength-multiplex optical communication system, which transmits and receives optical signals that have different wavelengths through multiplexing by using a wide wavelength band. On the above-mentioned background, a wideband light source that uses the ASE light of the rare earth-doped optical fiber is used as an incoherent light source for WDM and as an experimental light source for WDM system optical components.
By utilizing an erbium-doped fiber for the rare earth-doped optical fiber, a 1570-1610-nm band (1580-nm band) is applied to communications in addition to the 1530-to-1560-nm band (1550-nm band) applied conventionally, with which the wavelength band utilized for optical communications is being expanded. Accordingly, it is required for a component for optical communications in a wide wavelength band to operate in the wavelength band of 1530 to 1610 nm or higher band. In order to measure the loss wavelength characteristic of the component, there is demanded a high-output wideband light source to cover this wavelength band and expand the measurement dynamic range.
An example of the conventional wideband ASE light source is disclosed in Japanese Patent Laid-Open Publication No. 3-028830. As shown in FIG. 10, this light source is constructed of a pumping light source 101 for generating a pumping light of a prescribed wavelength, a rare earth-doped optical fiber 102 for generating an ASE light by the incident pumping light, a reflector 103 for reflecting the ASE light radiated from the rare earth-doped optical fiber and a multiplexer for uniting the rare earth-doped optical fiber 102, the pumping light source 101, an optical multiplexer 104 and an output terminal 105 with one another.
In this ASE light source, a pumping light Lp emitted from the light source 101 is transmitted through the optical multiplexer 104 and made incident on the rare earth-doped optical fiber 102. The rare earth-doped optical fiber is excited by the incident pumping light Lp and generates a forward light Lf in the forward direction and a backward light Lb in the backward direction as an ASE light. The forward light Lf is reflected on the reflector 103, made incident again on the rare earth-doped optical fiber 102 and made to pass together with the backward light Lb of the ASE light while being optically multiplexed in the optical multiplexer 104.
This optical multiplexer 104 has a function to optically combine lights of the wavelengths of the backward light Lb and the forward light Lf of the ASE light and to interrupt the light having the wavelength of the pumping light Lp. In this meaning, such a multiplexer should be a wavelength division multiplexer/demultiplexer, but is simply referred to as a multiplexer in this specification.
The ASE light of backward light Lb and forward light Lf, which has passed through the optical multiplexer 104, goes out of the output terminal 105.
As described above, by making the ASE light of forward light Lf reciprocally pass through the rare earth-doped optical fiber by reflecting the forward light Lf on the reflector 103, light of a higher intensity can be outputted. A fiber doped with erbium Er is able to emit light in a short wavelength hand of 1530 to 1570 nm as the backward light Lb of the ASE light and emit light in a longer wavelength band of not shorter than 1570 nm as the forward light Lf. Therefore, the light source of the Er-doped optical fiber can achieve a wideband light source by concurrently outputting the forward light Lf in the forward direction and the backward light Lb in the backward direction.
Moreover, although not shown in the aforementioned Japanese Patent Laid-Open Publication No. 3-028830, an optical isolator of a type that does not depend on a polarized wave is generally arranged in the stage before the output terminal 105. This optical isolator plays the role of removing a reflected return light to the rare earth-doped optical fiber 102 and produces the effect of suppressing parasitic oscillation which would be caused by the reflected return light and a reduction in the gain which would be caused by the multi-path reflection of the ASE light.
However, the conventional wideband ASE light source shown in FIG. 10 of the aforementioned Japanese Patent Laid-Open Publication No. 3-028830, which has a double-path structure of reflection, is therefore very sensitive to the reflected return light from the isolator. There has been the problem that ASE light ripples occur to cause an unstable output when the output of the pumping light source 101 is increased even with the optical isolator arranged and parasitic oscillation occur at a specific wavelength when the output is increased.
FIG. 11 is a graph showing the ripples of the ASE light from the wideband ASE light source. The ripples are periodic fluctuations (refer to B in the figure) of the output waveform with respect to the wavelength and caused by the amplification of a multi-path reflection interference phenomenon in the rare earth-doped optical fiber. The ripple amplitude as described above increases as the output of the pumping light source increases. The output of the wideband ASE light source becomes very unstable, and parasitic oscillation (refer to A in the figure) at a specific wavelength appears.
The Er-doped fiber used for the conventional wideband ASE light source has had a long length of, for example, 360 m and also unsuitable for compacting the device. The high power of the pumping light source and the long length of the Er-doped fiber have caused a drawback that the device has been expensive. The prior art has had the drawback that a compensating filter has been required for flattening the spectrum.